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In fact, a White House spokesman left the door open this week to a possible presidential endorsement in the primary season, while also telling reporters Obama believes his move to name Biden as his VP was the best political decision he had ever made.

An Obama endorsement would be a mixed blessing of course, given the president's lackluster public approval ratings.

Regardless, that prospect fuels speculation that those within the White House may see Hillary Clinton as a risky choice to be the nominee heading into next year.

Make no mistake, Obama has heaped plenty of praise on Clinton's performance as secretary of state. It's also true that Clinton exhibits a lot of strength, at this point, with healthy fundraising, a list of heavy-hitter supporters and a powerful campaign organization. But the FBI investigation into her use of a private email server is creating fallout.

Recent polls show Clinton's numbers are eroding in key primary states, hitting her especially on the issue of believability.

In addition, independent Bernie Sanders is generating strong enthusiasm among supporters at his campaign events. It adds to questions about Clinton's vulnerability and staying power, and whether the email issue, among others, will harm her prospects in the general election.

That sets up the buzz about Biden and whether he should jump in, even at this relatively late date.

According to the Washington Post, Biden is learning more toward running now than he was earlier in the summer.

Having run for the presidency twice before, Biden knows what's ahead. He clearly wants to do it. He's assessing the landscape with his advisers, fundraisers and his family.

What he has to decide is, first, how vulnerable Clinton is and, second, how electable he is. The last thing Biden wants to be known for is getting into the race, weakening Clinton who ends up with the nomination anyway, and thereby improving Republican chances of regaining the White House.

Author Kevin Riggs is an Emmy-winning former political reporter for KCRA-TV. He is now the station's political analyst and Senior Vice President at Randle Communications.